In many aircraft, the main propulsion engines not only provide propulsion for the aircraft, but may also be used to drive various other rotating components such as, for example, generators, compressors, and pumps, to thereby supply electrical and/or pneumatic power. However, when an aircraft is on the ground, its main engines may not be operating. Moreover, in some instances the main propulsion engines may not be capable of supplying the power needed for propulsion as well as the power to drive these other rotating components. Thus, many aircraft include an auxiliary power unit (APU) to supplement the main propulsion engines in providing electrical and/or pneumatic power. An APU may also be used to start the propulsion engines.
An APU is, in most instances, a gas turbine engine that includes a combustion system, a power turbine, and a compressor. During operation of the APU, the compressor draws in ambient air, compresses it, and supplies compressed air to the combustion system. The combustion system receives fuel from a fuel source and the compressed air from the compressor, and supplies high-energy combusted air to the power turbine, causing it to rotate. The power turbine includes a shaft that may be used to drive a generator for supplying electrical power, and to drive its own compressor and/or an external load compressor.
Typically, APUs and associated cooling systems are mounted in a compartment in the aft section of the aircraft, at or near the aircraft tailcone section. The aircraft typically includes at least one air inlet and one exhaust outlet to allow sufficient air flow into and through the APU. Many aircraft include more than one air inlet to support APU operations. One air inlet may provide air flow to the APU compressor, while the other air inlet may provide air flow for oil and compartment cooling. More specifically, the APU is typically lubricated with air-cooled lubricant, such as oil, that flows through an oil cooler, and across which a flow of cooling air is also directed. This flow of cooling air may also provide air cooling for the APU and the APU compartment.
Many APU systems include passive, eductor driven cooling systems to provide both oil and APU compartment cooling. Passive, eductor driven cooling systems typically include an inlet scoop that may be positioned inside the APU inlet duct. The scoop diverts a portion of the air flow in the inlet duct through an outlet in the wall of the inlet duct. These systems typically need a positive pressure gradient across the cooling system for optimum operation in flight. To achieve this positive pressure gradient across the cooling system is configured to develop a ram pressure recovery higher than the exhaust exit pressure. Thus, many aircraft include a ram air inlet door that is movable between an open position and a closed position. In the open position, the inlet door provides excellent ram air recovery. In the closed position, the inlet door protects the APU from foreign objects when not in use and/or during ground movement. In addition, during flight, the closed door prevents air circulation through the APU when the APU is in not in use. Such air circulation through the APU can cause aircraft drag, and may undesirably cause the APU to windmill uncontrollably.
Although the above-described configuration is generally safe, robust, and reliable, it does suffer certain drawbacks. For example, when the inlet door is in the closed position, the APU compartment and inlet duct are isolated by the door inner surface. However, because the cooling scoop is inside the inlet duct, in the highly unlikely event of a compartment fire, if a portion of the cooling inlet wall bums through, the fire can enter the engine inlet duct violating fire isolation regulations. Although separate and individually operable and isolable inlets could be used, such a configuration can undesirably increase overall system weight and cost since more than one actuator is typically used.
Hence, there is a need for an APU compartment air inlet system that can be sealed when the APU is not operating during in-flight conditions and/or that allows the APU inlet and cooling system inlets to be isolated from each other and/or that does not undesirably increase overall system weight and/or cost. The present invention addresses one or more of these needs.